This man page should serve as a first tutorial on the indexing and
threading features of PDL.

Like all vectorized languages, PDL automates looping over arrays using
a variant of mathematical vector notation. The automatic looping is called
"threading", in part because ultimately PDL will implement parallel processing
to speed up the loops.

A lot of the flexibility and power of PDL relies on the indexing and threading
features of the Perl extension. Indexing allows access to the data of a piddle
in a very flexible way. Threading provides efficient vectorization of simple
operations.

The values of a piddle are stored compactly as typed values in a single block of memory,
not (as in a normal Perl list-of-lists) as individual Perl scalars.

In the sections that follow many "methods" are called out -- these are Perl operators
that apply to PDLs. From the perldl (or pdl2) shell, you
can find out more about each method by typing "?" followed by the method name.

A piddle (PDL variable), in general, is an N-dimensional array where N can be
0 (for a scalar), 1 (e.g. for a sound sample), or higher values for images
and more complex structures. Each dimension of the piddle has a positive
integer size. The perl interpreter treats each piddle as a special type of
Perl scalar (a blessed Perl object, actually -- but you don't have to know that
to use them) that can be used anywhere you can put a normal scalar.

You can access the dimensions of a piddle as a Perl list and otherwise determine
the size of a piddle with several methods. The important ones are:

PDL maintains a notion of "dataflow" between a piddle and indexed subfields of
that piddle. When you produce an indexed subfield or single element of a parent
piddle, the child and parent remain attached until you manually disconnect them.
This lets you represent the same data different ways within your code -- for example,
you can consider an RGB image simultaneously as a collection of (R,G,B) values
in a 3 x 1000 x 1000 image, and as three separate 1000 x 1000 color planes stored in
different variables. Modifying any of the variables changes the underlying memory, and
the changes are reflected in all representations of the data.

There are two important methods that let you control dataflow connections between a
child and parent PDL:

Most PDL operations act on the first few dimensions of their piddle arguments. For
example, sumover sums all elements along the first dimension in the list (dimension 0).
If you feed in a three-dimensional piddle, then the first dimension is considered the
"active" dimension and the later dimensions are "thread" dimensions because they are simply
looped over. There are several ways to transpose or re-order the dimension list of a PDL.
Those techniques are very fast since they don't touch the underlying data, only change the
way that PDL accesses the data. The main dimension ordering functions are:

A lot of the flexibility and power of PDL relies on the indexing and
looping features of the Perl extension. Indexing allows access to the
data of a pdl object in a very flexible way. Threading provides
efficient implicit looping functionality (since the loops are
implemented as optimized C code).

Pdl objects (later often called "pdls") are Perl objects that
represent multidimensional arrays and operations on those. In contrast
to simple Perl @x style lists the array data is compactly stored in
a single block of memory thus taking up a lot less memory and enabling
use of fast C code to implement operations (e.g. addition,
etc) on pdls.

Central to many of the indexing capabilities of PDL are the relation of
"parent" and "child" between pdls. Many of the indexing commands
create a new pdl from an existing pdl. The new pdl is the "child"
and the old one is the "parent". The data of the new pdl is defined by a
transformation that specifies how to generate (compute) its data from
the parent's data. The relation between the child pdl and its parent
are often bidirectional, meaning that changes in the child's data are
propagated back to the parent. (Note: You see, we are aiming in our
terminology already towards the new dataflow features. The kind of
dataflow that is used by the indexing commands (about which you will
learn in a minute) is always in operation, not only when you have
explicitly switched on dataflow in your pdl by saying $a->doflow. For
further information about data flow check the dataflow man page.)

Another way to interpret the pdls created by our indexing commands is
to view them as a kind of intelligent pointer that points back to some
portion or all of its parent's data. Therefore, it is not surprising
that the parent's data (or a portion of it) changes when manipulated
through this "pointer". After these introductory remarks that
hopefully prepared you for what is coming (rather than confuse you too
much) we are going to dive right in and start with a description of
the indexing commands and some typical examples how they might be used
in PDL programs. We will further illustrate the pointer/dataflow
analogies in the context of some of the examples later on.

There are two different implementations of this ``smart pointer''
relationship: the first one, which is a little slower but works
for any transformation is simply to do the transformation forwards
and backwards as necessary. The other is to consider the child piddle
a ``virtual'' piddle, which only stores a pointer to the parent
and access information so that routines which use the child piddle
actually directly access the data in the parent.
If the virtual piddle is given to a routine which cannot use it,
PDL transparently physicalizes the virtual piddle before letting
the routine use it.

Currently (1.94_01) all transformations which are ``affine'',
i.e. the indices of the data item in the parent piddle are determined
by a linear transformation (+ constant) from the indices of the
child piddle result in virtual piddles. All other indexing
routines (e.g. ->index(...)) result in physical piddles.
All routines compiled by PP can accept affine piddles (except
those routines that pass pointers to external library functions).

Note that whether something is affine or not does not affect the semantics
of what you do in any way: both

$a->index(...) .= 5;
$a->slice(...) .= 5;

change the data in $a. The affinity does, however, have a significant
impact on memory usage and performance.

Probably the most important application of the concept of parent/child
pdls is the representation of rectangular slices of a physical pdl by
a virtual pdl. Having talked long enough about concepts let's get more
specific. Suppose we are working with a 2D pdl representing a 5x5
image (its unusually small so that we can print it without filling
several screens full of digits ;).

[ here it might be appropriate to quickly talk about the
help vars command
that provides information about pdls in the interactive
perldl or pdl2 shell that comes with PDL.
]

Now suppose we want to create a 1-D pdl that just references
one line of the image, say line 2; or a pdl that represents all even
lines of the image (imagine we have to deal with even and odd frames
of an interlaced image due to some peculiar behaviour of our frame
grabber). As another frequent application of slices we might want to
create a pdl that represents a rectangular region of the image with
top and bottom reversed. All these effects (and many more) can be
easily achieved with the powerful slice function:

All three "child" pdls are children of $im or in the other (largely
equivalent) interpretation pointers to data of $im. Operations on
those virtual pdls access only those portions of the data as specified
by the argument to slice. So we can just print line 2:

pdl> p $line
[10 11 12 13 14]

Also note the difference in the "Flow State" of $area above
and below:

Note how assignment operations on the child virtual pdls change the
parent physical pdl and vice versa (however, the basic "=" assignment
doesn't, use ".=" to obtain that effect. See below for the reasons).
The virtual child pdls are
something like "live links" to the "original" parent pdl. As
previously said, they can be thought of to work similar to a
C-pointer. But in contrast to a C-pointer they carry a lot more
information. Firstly, they specify the structure of the data they
represent (the dimensionality of the new pdl) and secondly, specify
how to create this structure from its parents data (the way this works
is buried in the internals of PDL and not important for you to know
anyway (unless you want to hack the core in the future or would like
to become a PDL guru in general (for a definition of this strange
creature see PDL::Internals)).

The previous examples have demonstrated typical usage of the slice
function. Since the slicing functionality is so important here is an
explanation of the syntax for the string argument to slice:

$vpdl = $a->slice('ind0,ind1...')

where ind0 specifies what to do with index No 0 of the pdl $a,
etc. Each element of the comma separated list can have one of the
following forms:

Use only index n. This dimension is removed from the
resulting pdl (relying on the fact that a dimension of size 1 can always be
removed). The distinction between this case and the previous one
becomes important in assignments where left and right hand side have to
have appropriate dimensions.

Take the range of indices from n1 to n2 or (second form)
take the range of indices from n1 to n2 with step
n3. An example for the use of this format is the previous
definition of the sub-image composed of even lines.

pdl> $even = $im->slice(':,1:-1:2')

This example also demonstrates that negative indices work like they do
for normal Perl style arrays by counting backwards from the end of the
dimension. If n2 is smaller than n1 (in the example -1 is
equivalent to index 4) the elements in the virtual pdl are effectively
reverted with respect to its parent.

Add a dummy dimension. The size of this dimension will be 1 by default
or equal to n if the optional numerical argument is given.

Now, this is really something a bit strange on first sight. What is a
dummy dimension? A dummy dimension inserts a dimension where there
wasn't one before. How is that done ? Well, in the case of the new
dimension having size 1 it can be easily explained by the way in which
you can identify a vector (with m elements) with an (1,m) or (m,1)
matrix. The same holds obviously for higher dimensional objects. More
interesting is the case of a dummy dimensions of size greater than one
(e.g. slice('*5,:')). This works in the same way as a call to the
dummy function creates a new dummy dimension.
So read on and check
its explanation below.

[Not yet implemented ??????]
With an argument like this you make generalised diagonals. The
diagonal will be dimension no. i of the new output pdl and (if
optional part in brackets specified) will extend along the range of
indices specified of the respective parent pdl's dimension. In general
an argument like this only makes sense if there are other arguments
like this in the same call to slice. The part in brackets is optional
for this type of argument. All arguments of this type that specify the
same target dimension i have to relate to the same number of
indices in their parent dimension. The best way to explain it is probably to
give an example, here we make a pdl that refers to the elements along
the space diagonal of its parent pdl (a cube):

$cube = zeroes(5,5,5);
$sdiag = $cube->slice('(=0),(=0),(=0)');

The above command creates a virtual pdl that represents the diagonal
along the parents' dimension no. 0, 1 and 2 and makes its dimension 0
(the only dimension) of it. You use the extended syntax if the
dimension sizes of the parent dimensions you want to build the
diagonal from have different sizes or you want to reverse the sequence
of elements in the diagonal, e.g.

The previous examples have already shown that virtual pdls can be used
to operate on or access portions of data of a parent pdl. They can
also be used as lvalues in assignments (as the use of ++ in some of
the examples above has already demonstrated). For explicit assignments
to the data represented by a virtual pdl you have to use the
overloaded .= operator (which in this context we call propagated
assignment). Why can't you use the normal assignment operator =?

Well, you definitely still can use the '=' operator but it wouldn't do
what you want. This is due to the fact that the '=' operator cannot be
overloaded in the same way as other assignment operators. If we tried
to use '=' to try to assign data to a portion of a physical pdl
through a virtual pdl we wouldn't achieve the desired effect (instead
the variable representing the virtual pdl (a reference to a
blessed thingy) would after the assignment just contain the reference
to another blessed thingy which would behave to future assignments as
a "physical" copy of the original rvalue [this is actually not yet
clear and subject of discussions in the PDL developers mailing
list]. In that sense it would break the connection of the pdl to the
parent [ isn't this behaviour in a sense the opposite of what happens in
dataflow, where .= breaks the connection to the parent? ].

for the assignment above (the zero is converted to a scalar piddle,
with no dimensions so it can be assigned to any piddle).

A nice feature in recent perl versions is lvalue subroutines
(i.e., versions 5.6.x and higher including all perls currently
supported by PDL). That allows one to use the slicing syntax
on both sides of the assignment:

pdl> $im->slice(':,(2)') .= zeroes(5)->xvals->float

Related to the lvalue sub assignment feature is a little trap
for the unwary: recent perls introduced a "feature" which breaks
PDL's use of lvalue subs for slice assignments when running under
the perl debugger, perl -d. Under the debugger, the above
usage gives an error like:
Can't return a temporary from lvalue subroutine...
So you must use syntax like this:

pdl> ($pdl = $im->slice(':,(2)')) .= zeroes(5)->xvals->float

which works both with and without the debugger but is arguably
clumsy and awkward to read.

Note that there can be a problem with assignments like this when
lvalue and rvalue pdls refer to overlapping portions of data in the
parent pdl:

# revert the elements of the first line of $a
($tmp = $a->slice(':,(1)')) .= $a->slice('-1:0,(1)');

Currently, the parent data on the right side of the assignments is not
copied before the (internal) assignment loop proceeds. Therefore, the
outcome of this assignment will depend on the sequence in which
elements are assigned and almost certainly not do what you
wanted. So the semantics are currently undefined for now and liable
to change anytime. To obtain the desired behaviour, use

($tmp = $a->slice(':,(1)')) .= $a->slice('-1:0,(1)')->copy;

which makes a physical copy of the slice or

($tmp = $a->slice(':,(1)')) .= $a->slice('-1:0,(1)')->sever;

which returns the same slice but severs the connection of the slice
to its parent.

Having talked extensively about the
slice function it should be
noted that this is not the only PDL indexing function. There
are additional indexing functions which are also useful
(especially in the context of threading which we will talk about
later). Here are a list and some examples how to use them.

inserts a dummy dimension of the size you specify (default 1) at the
chosen location. You can't wait to hear how that is achieved? Well,
all elements with index (X,x,Y) (0<=x<size_of_dummy_dim) just map to
the element with index (X,Y) of the parent pdl (where X and Y refer to
the group of indices before and after the location where the dummy
dimension was inserted.)

This example calculates the x coordinate of the centroid of an
image (later we will learn that we didn't actually need the dummy
dimension thanks to the magic of implicit threading; but using dummy
dimensions the code would also work in a thread-less world; though once
you have worked with PDL threads you wouldn't want to live without
them again).

Let's explain how that works in a little more detail. First, the
product:

$xvs = xvals(zeroes($xd));
print $xvs->dummy(1,$yd); # repeat the line $yd times
$prod = $im*xvs->dummy(1,$yd); # form the pixel-wise product with
# the repeated line of x-values

The rest is then summing the results of the pixel-wise product together
and normalizing with the sum of all pixel values in the original image
thereby calculating the x-coordinate of the "center of mass" of the
image (interpreting pixel values as local mass) which is known as the
centroid of an image.

Next is a (from the point of view of memory consumption) very
cheap conversion from grey-scale to RGB, i.e. every pixel holds now a
triple of values instead of a scalar. The three values in the triple
are, fortunately, all the same for a grey image, so that our trick
works well in that it maps all the three members of the triple to the
same source element:

# a cheap grey-scale to RGB conversion
$rgb = $grey->dummy(0,3)

Unfortunately this trick cannot be used to convert your old B/W
photos to color ones in the way you'd like. :(

Note that the memory usage of piddles with dummy dimensions
is especially sensitive to the internal representation. If the piddle
can be represented as a virtual affine (``vaffine'') piddle,
only the control structures are stored. But if $b in

$a = zeroes(10000);
$b = $a->dummy(1,10000);

is made physical by some routine, you will find that the memory usage
of your program has suddenly grown by 100Mb.

replaces two dimensions (which have to be of equal size) by one
dimension that references all the elements along the "diagonal" along
those two dimensions. Here, we have two examples which should appear
familiar to anyone who has ever done some linear algebra. Firstly,
make a unity matrix:

(Did you notice how we used the slice function to revert the sequence
of lines before setting the diagonal of the new child, thereby setting
the cross diagonal of the parent ?) Or a mapping from the space of
diagonal matrices to the field over which the matrices are defined,
the trace of a matrix:

$prod should now be pretty close to the unity matrix if $a is an
orthogonal matrix. Often xchg will be used in the context of threading
but more about that later.

mv works in a similar fashion. It moves a dimension (specified by
its number in the parent) to a new position in the new child pdl:

$b = $a->mv(4,0); # make the 5th dimension of $a the first in the
# new child $b

The difference between xchg and mv is that xchg only changes
the position of two dimensions with each other, whereas mv
inserts the first dimension to the place of second, moving the other
dimensions around accordingly.

collapses several dimensions into one. Its only argument specifies how
many dimensions of the source pdl should be collapsed (starting from
the first). An (admittedly unrealistic) example is a 3D pdl which
holds data from a stack of image files that you have just read
in. However, the data from each image really represents a 1D time
series and has only been arranged that way because it was digitized
with a frame grabber. So to have it again as an array of time
sequences you say

As you might have noticed in some of the examples above calls to the
indexing functions can be nicely chained since all of these functions
return a newly created child object. However, when doing extensive
index manipulations in a chain be sure to keep track of what you are
doing, e.g.

$a->xchg(0,1)->mv(0,4)

moves the dimension 1 of $a to position 4 since when the
second command is executed the original dimension 1 has been moved
to position 0 of the new child that calls the mv function. I think
you get the idea (in spite of my convoluted explanations).

A sublety related to indexing is the assignment to pdls containing dummy
dimensions of size greater than 1. These assignments (using .=) are
forbidden since several elements of the lvalue pdl point to the same
element of the parent. As a consequence the value of those parent
elements are potentially ambiguous and would depend on the sequence in
which the implementation makes the assignments to elements. Therefore,
an assignment like this:

$a = pdl [1,2,3];
$b = $a->dummy(1,4);
$b .= yvals(zeroes(3,4));

can produce unexpected results and the results are explicitly
undefined by PDL because when PDL gets parallel computing
features, the current result may well change.

From the point of view of dataflow the introduction of
greater-size-than-one dummy dimensions is regarded as an irreversible
transformation (similar to the terminology in thermodynamics) which
precludes backward propagation of assignment to a parent (which you had
explicitly requested using the .= assignment). A similar problem to
watch out for occurs in the context of threading where sometimes
dummy dimensions are created implicitly during the thread loop (see below).

XXXXX being memory efficient
XXXXX in the context of threading
XXXXX very flexible and powerful way of accessing portions of pdl data
(in much more general way than sec, etc allow)
XXXXX efficient implementation
XXXXX difference to section/at, etc.

In the previous paragraph on indexing we have already mentioned the
term occasionally but now its really time to talk explicitly about
"threading" with pdls. The term threading has many different meanings in
different fields of computing. Within the framework of PDL it could
probably be loosely defined as an implicit looping
facility. It is implicit because you don't specify anything like
enclosing for-loops but rather the loops are automatically (or
'magically') generated by PDL based on the dimensions of the pdls
involved. This should give you a first idea why the index/dimension
manipulating functions you have met in the previous paragraphs are
especially important and useful in the context of threading.
The other ingredient for threading (apart from the pdls involved) is
a function that is threading aware (generally, these are
PDL::PP compiled functions) and that the pdls are "threaded" over.
So much about the terminology and now let's try to shed some light on what it
all means.

There are two slightly different variants of threading. We start
with what we call "implicit threading". Let's pick a practical example
that involves looping of a function over many elements of a
pdl. Suppose we have an RGB image that we want to convert to
grey-scale. The RGB image is represented by a 3-dim pdl im(3,x,y) where
the first dimension contains the three color components of each pixel and x
and y are width and height of the image, respectively. Next we need to
specify how to convert a color-triple at a given pixel into a
grey-value (to be a realistic example it should represent the relative
intensity with which our color insensitive eye cells would detect that
color to achieve what we would call a natural conversion from color to
grey-scale). An approximation that works quite well is to compute the
grey intensity from each RGB triplet (r,g,b) as a weighted sum

where the last form indicates that we can write this as an inner
product of the 3-vector comprising the weights for red, green and blue
components with the 3-vector containing the color
components. Traditionally, we might have written a function like the
following to process the whole image:

Now we write the same using threading (noting that inner is a threading
aware function defined in the PDL::Primitive package)

$grey = inner($im,pdl([77,150,29]/256));

We have ended up with a one-liner that automatically
creates the pdl $grey with the right number and size of dimensions and
performs the loops automatically (these loops are implemented as fast C code
in the internals of PDL).
Well, we
still owe you an explanation how this 'magic' is achieved.

The first thing to note is that every function that is threading aware
(these are without exception functions compiled from concise
descriptions by PDL::PP, later just called PP-functions) expects a
defined (minimum) number of dimensions (we call them core dimensions)
from each of its pdl arguments. The inner function
expects two one-dimensional (input) parameters from which it calculates a
zero-dimensional (output) parameter. We write that symbolically as
inner((n),(n),[o]()) and call it inner's signature, where n
represents the size of that dimension. n being equal in the first and
second parameter means that those dimensions have to be of equal size
in any call. As a different example take the outer product which takes
two 1D vectors to generate a 2D matrix, symbolically written as
outer((n),(m),[o](n,m)). The [o] in both examples indicates that
this (here third) argument is an output argument. In the latter
example the dimensions of first and second argument don't have to
agree but you see how they determine the size of the two dimensions of
the output pdl.

Here is the point when threading finally enters the game. If you call
PP-functions with pdls that have more than the required core
dimensions the first dimensions of the pdl arguments are used as the
core dimensions and the additional extra dimensions are threaded
over. Let us demonstrate this first with our example above

$grey = inner($im,$w); # w is the weight vector from above

In this case $w is 1D and so supplied just the core dimension, $im is
3D, more specifically (3,x,y). The first dimension (of size 3) is the
required core dimension that matches (as required by inner) the first
(and only) dimension of $w. The second dimension is the first thread
dimension (of size x) and the third is here the second thread
dimension (of size y). The output pdl is automatically created (as
requested by setting $grey to "null" prior to invocation). The output
dimensions are obtained by appending the loop dimensions (here
(x,y)) to the core output dimensions (here 0D) to yield the final
dimensions of the auto-created pdl (here 0D+2D=2D to yield a 2D
output of size (x,y)).

So the above command calls the core functionality that computes the inner
product of two 1D vectors x*y times with $w and all 1D slices of the
form (':,(i),(j)') of $im and sets the respective elements of the
output pdl $grey(i,j) to the result of each computation. We could
write that symbolically as

But this is done automatically by PDL without writing any explicit
Perl loops. We see that the command really creates an output pdl with
the right dimensions and sets the elements indeed to the result of the
computation for each pixel of the input image.

When even more pdls and extra dimensions are involved things get a bit
more complicated. We will first give the general rules how the thread
dimensions depend on the dimensions of input pdls enabling you to
figure out the dimensionality of an auto-created output pdl (for any
given set of input pdls and core dimensions of the PP-function in
question). The general rules will most likely appear a bit confusing
on first sight so that we'll set out to illustrate the usage with a set
of further examples (which will hopefully also demonstrate that there
are indeed many practical situations where threading comes in extremely
handy).

Before we point out the other technical details of threading, please
note this call for programming discipline when using threading:

In order to preserve human readability, PLEASE comment any nontrivial
expression in your code involving threading. Most importantly, for
any subroutine, include information at the beginning about what you
expect the dimensions to represent (or ranges of dimensions).

As a warning, look at this undocumented function and try to guess what
might be going on:

There are a couple of rules that allow you to figure out number and
size of loop dimensions (and if the size of your input pdls comply
with the threading rules). Dimensions of any pdl argument are broken
down into two groups in the following: Core dimensions (as defined by
the PP-function, see Appendix B for a list of PDL primitives) and
extra dimensions which comprises all remaining dimensions of that
pdl. For example calling a function func with the signature
func((n,m),[o](n)) with a pdl a(2,4,7,1,3) as f($a,($o = null))
results in the semantic splitting of a's dimensions into:
core dimensions (2,4) and extra dimensions (7,1,3).

The size of each of the loop dimensions is derived from the size
of the respective dimensions of the pdl arguments. The size of a
loop dimension is given by the maximal size found in any of
the pdls having this extra dimension.

For all pdls that have a given extra dimension the size must be
equal to the size of the loop dimension (as determined by the
previous rule) or 1; otherwise you raise a runtime exception. If the
size of the extra dimension in a pdl is one it is implicitly treated
as a dummy dimension of size equal to that loop dim size when
performing the thread loop.

If output auto-creation is used (by setting the relevant pdl to
PDL->null before invocation) the number of dimensions of the created
pdl is equal to the sum of the number of core output dimensions +
number of loop dimensions. The size of the core output dimensions is
derived from the relevant dimension of input pdls (as specified in the
function definition) and the sizes of the other dimensions are equal
to the size of the loop dimension it is derived from. The
automatically created pdl will be physical (unless dataflow is in
operation).

In this context, note that you can run into the problem with
assignment to pdls containing greater-than-one dummy dimensions (see above).
Although your output pdl(s) didn't contain any dummy dimensions in the
first place they may end up with implicitly created dummy dimensions
according to R4.

As an example, suppose we have a (here unspecified) PP-function with
the signature:

func((m,n),(m,n,o),(m),[o](m,o))

and you call it with 3 pdls a(5,3,10,11),
b(5,3,2,10,1,12), and c(5,1,11,12) as

func($a,$b,$c,($d=null))

then the number of loop dimensions is 3 (by R0+R1 from $b and $c) with
sizes (10,11,12) (by R2); the two output core dimensions are (5,2)
(from the signature of func) resulting in a 5-dimensional output pdl
$c of size (5,2,10,11,12) (see R5) and (the automatically created) $d
is derived from ($a,$b,$c) in a way that can be expressed in pdl
pseudo-code as

If we analyze the color to grey-scale conversion again with these rules
in mind we note another great advantage of implicit threading.
We can call the conversion with a pdl representing a pixel (im(3)),
a line of rgb pixels (im(3,x)), a proper color image (im(3,x,y)) or a
whole stack of RGB images (im(3,x,y,z)). As long as $im is of the form
(3,...) the automatically created output pdl will contain the right
number of dimensions and contain the intensity data as we expect it
since the loops have been implicitly performed thanks to implicit
threading. You can easily convince yourself that calling with a color
pixel $grey is 0D, with a line it turns out 1D grey(x), with an image
we get grey(x,y) and finally we get a converted image stack grey(x,y,z).

Let's fill these general rules with some more life by going through a
couple of further examples. The reader may try to figure out equivalent
formulations with explicit for-looping and compare the flexibility of
those routines using implicit threading to the explicit
formulation. Furthermore, especially when using several thread
dimensions it is a useful exercise to check the relative speed
by doing some benchmark tests (which we still have to do).

First in the row is a slightly reworked centroid example, now coded
with threading in mind.

This will actually work for $im being one, two, three, and higher
dimensional. If $im is one-dimensional it's just an ordinary product
(in the sense that every element of $im is multiplied with the
respective element of xvals(...)), if $im has more dimensions further
threading is done by adding appropriate dummy dimensions to xvals(...)
according to R4.
More importantly, the two sumover operations show
a first example of how to make use of the dimension manipulating
commands. A quick look at sumover's signature will remind you that
it will only "gobble up" the first dimension of a given input pdl. But
what if we want to really compute the sum over all elements of the
first two dimensions? Well, nothing keeps us from passing a virtual
pdl into sumover which in this case is formed by clumping the first
two dimensions of the "parent pdl" into one. From the point of view of
the parent pdl the sum is now computed over the first two dimensions,
just as we wanted, though sumover has just done the job as specified
by its signature. Got it ?

Another little finesse of writing the code like that: we intentionally
used sumover($pdl->clump(2)) instead of sum($pdl) so that we can
either pass just an image (x,y) or a stack of images (x,y,t) into this
routine and get either just one x-coordiante or a vector of
x-coordinates (of size t) in return.

Another set of common operations are what one could call "projection
operations". These operations take a N-D pdl as input and return a
(N-1)-D "projected" pdl. These operations are often performed with
functions like sumover,
prodover, minimum and
maximum.
Using again images as examples we might want to calculate
the maximum pixel value for each line of an image or image stack. We
know how to do that

# maxima of lines (as function of line number and time)
maximum($stack,($ret=null));

But what if you want to calculate maxima per column when implicit
threading always applies the core functionality to the first dimension
and threads over all others? How can we achieve that instead the
core functionality is applied to the second dimension and threading is
done over the others. Can you guess it? Yes, we make a virtual pdl
that has the second dimension of the "parent pdl" as its first
dimension using the mv command.

# maxima of columns (as function of column number and time)
maximum($stack->mv(1,0),($ret=null));

and calculating all the sums of sub-slices over the third dimension
is now almost too easy

# sums of pixels in time (assuming time is the third dim)
sumover($stack->mv(2,0),($ret=null));

Finally, if you want to apply the operation to all elements (like max over
all elements or sum over all elements) regardless of the dimensions of
the pdl in question clump comes in handy. As an example look at the
definition of sum (as defined in Ufunc.pm):

To finish the examples in this paragraph here is a function to create
an RGB image from what is called a palette image. The palette image
consists of two parts: an image of indices into a color lookup table
and the color lookup table itself. [ describe how it works ] We
are going to use a PP-function we haven't encoutered yet in the previous
examples. It is the aptly named index function, signature
((n),(),[o]()) (see Appendix B) with the core functionality that
index(pdl (0,2,4,5),2,($ret=null)) will return the element with index
2 of the first input pdl. In this case, $ret will contain the value 4.
So here is the example:

# a threaded index lookup to generate an RGB, or RGBA or YMCK image
# from a palette image (represented by a lookup table $palette and
# an color-index image $im)
# you can say just dummy(0) since the rules of threading make it fit
pdl> index($palette->xchg(0,1),
$im->long->dummy(0,($palette->dim)[0]),
($res=null));

Let's go through it and explain the steps involved. Assuming we are
dealing with an RGB lookup-table $palette is of size (3,x). First we
exchange the dimensions of the palette so that looping is done over
the first dimension of $palette (of size 3 that represent r, g, and b
components). Now looking at $im, we add a dummy dimension of size
equal to the length of the number of components (in the case we are
discussing here we could have just used the number 3 since we have 3
color components). We can use a dummy dimension since for red, green
and blue color components we use the same index from the original
image,
e.g.
assuming a certain pixel of $im had the value 4 then the
lookup should produce the triple

[palette(0,4),palette(1,4),palette(2,4)]

for the new red, green and
blue components of the output image. Hopefully by now you have some
sort of idea what the above piece of code is supposed to do (it is
often actually quite complicated to describe in detail how a piece of
threading code works; just go ahead and experiment a bit to get a
better feeling for it).

If you have read the threading rules carefully, then you might have
noticed that we didn't have to explicitly state the size of the dummy
dimension that we created for $im; when we create it with size 1 (the
default) the rules of threading make it automatically fit to the
desired size (by rule R3, in our example the size would be 3 assuming
a palette of size (3,x)). Since situations like this do occur often in
practice this is actually why rule R3 has been introduced (the part
that makes dimensions of size 1 fit to the thread loop dim size). So
we can just say

pdl> index($palette->xchg(0,1),$im->long->dummy(0),($res=null));

Again, you can convince yourself that this routine will create the
right output if called with a pixel ($im is 0D), a line ($im is 1D),
an image ($im is 2D), ..., an RGB lookup table (palette is (3,x)) and
RGBA lookup table (palette is (4,x), see e.g. OpenGL). This
flexibility is achieved by the rules of threading which are made to do
the right thing in most situations.

To wrap it all up once again, the general idea is as follows. If you
want to achieve looping over certain dimensions and have the core functionality
applied to another specified set of dimensions you use
the dimension manipulating commands to create a (or several)
virtual pdl(s) so that from the point of view of the parent
pdl(s) you get what you want (always having the signature of the
function in question and R1-R5 in mind!). Easy, isn't it ?

At this point we have to divert to some technical detail that has to
do with the general calling conventions of PP-functions and the
automatic creation of output arguments.
Basically, there are two ways of invoking pdl routines, namely

$result = func($a,$b);

and

func($a,$b,$result);

If you are only using implicit threading then the output variable can
be automatically created by PDL. You flag that to the PP-function by
setting the output argument to a special kind of pdl that is returned
from a call to the function PDL->null that returns an essentially
"empty" pdl (for those interested in details there is a flag
in the C pdl structure for this). The dimensions
of the created pdl are determined by the rules of implicit
threading: the first dimensions are the core output dimensions to
which the threading dimensions are appended (which are in turn
determined by the dimensions of the input pdls as described above).
So you can say

func($a,$b,($result=PDL->null));

or

$result = func($a,$b)

which are exactly equivalent.

Be warned that you can not use output auto-creation when using
explicit threading (for reasons explained in the following section on
explicit threading, the second variant of threading).

In "tight" loops you probably want to avoid the implicit creation of a
temporary pdl in each step of the loop that comes along with the
"functional" style but rather say

# create output pdl of appropriate size only at first invocation
$result = null;
for (0...$n) {
func($a,$b,$result); # in all but the first invocation $result
func2($b); # is defined and has the right size to
# take the output provided $b's dims don't change
twiddle($result,$a); # do something from $result to $a for iteration
}

The take-home message of this section once more: be aware of the
limitation on output creation when using explicit threading.

Having so far only talked about the first flavour of threading it is
now about time to introduce the second variant. Instead of shuffling
around dimensions all the time and relying on the rules of implicit
threading to get it all right you sometimes might want to specify in a
more explicit way how to perform the thread loop. It is probably not
too surprising that this variant of the game is called explicit threading.
Now, before we create the wrong impression: it is not
either implicit or explicit; the two flavours do mix. But more
about that later.

The two most used functions with explicit threading are
thread
and unthread.
We start with an example that illustrates typical
usage of the former:

[ # ** this is the worst possible example to start with ]
# but can be used to show that $mat += $line is different from
# $mat->thread(0) += $line
# explicit threading to add a vector to each column of a matrix
pdl> $mat = zeroes(4,3)
pdl> $line = pdl (3.1416,2,-2)
pdl> ($tmp = $mat->thread(0)) += $line

In this example, $mat->thread(0) tells PDL that you want the second
dimension of this pdl to be threaded over first leading to a thread
loop that can be expressed as

for (j=0; j<3; j++) {
for (i=0; i<4; i++) {
mat(i,j) += src(j);
}
}

thread takes a list of numbers as arguments which explicitly
specify which dimensions to thread over first. With the introduction
of explicit threading the dimensions of a pdl are conceptually split into
three different groups the latter two of which we have already
encountered: thread dimensions, core dimensions and extra dimensions.

Conceptually, it is best to think of those dimensions of a pdl that
have been specified in a call to thread as being taken away from
the set of normal dimensions and put on a separate stack. So assuming
we have a pdl a(4,7,2,8) saying

$b = $a->thread(2,1)

creates a new virtual pdl of dimension b(4,8) (which we call the
remaining dims) that also has 2 thread dimensions of size (2,7). For
the purposes of this document we write that symbolically as
b(4,8){2,7}. An important difference to the previous examples where
only implicit threading was used is the fact that the core dimensions
are matched against the remaining dimensions which are not
necessarily the first dimensions of the pdl. We will now specify how
the presence of thread dimensions changes the rules R1-R5 for
thread loops (which apply to the special case where none of the pdl
arguments has any thread dimensions).

Core dimensions are matched against the first n remaining dimensions
of the pdl argument (note the difference to R1). Any
further remaining dimensions are extra dimensions and are used
to determine the implicit loop dimensions.

The total number of loop dimensions is equal to the sum of
explicit loop dimensions and implicit loop dimensions. In the
thread loop, explicit loop dimensions are threaded over first
followed by implicit loop dimensions.

The size of each of the loop dimensions is derived from the size of
the respective dimensions of the pdl arguments. It is given by the
maximal size found in any pdls having this thread dimension (for
explicit loop dimensions) or extra dimension (for
implicit loop dimensions).

This rule applies to any explicit loop dimension as well as any
implicit loop dimension. For all pdls that have a given
thread/extra dimension the size must be equal to the size of the
respective explicit/implicit loop dimension or 1; otherwise you
raise a runtime exception. If the size of a thread/extra dimension
of a pdl is one it is implicitly treated as a dummy
dimension of size equal to the explicit/implicit loop dimension.

If a pdl doesn't have a thread/extra dimension that corresponds to
an explicit/implicit loop dimension, in the thread loop this
pdl is treated as if having a dummy dimension of size equal to the
size of that loop dimension.

Output auto-creation cannot be used if any of the pdl arguments has any
thread dimensions. Otherwise R5 applies.

The same restrictions apply with regard to implicit dummy dimensions
(created by application of T4) as already mentioned in the section
on implicit threading: if any of the output pdls has an (explicit or
implicitly created) greater-than-one dummy dimension a runtime
exception will be raised.

Let us demonstrate these rules at work in a generic case.
Suppose we have a (here unspecified) PP-function with
the signature:

func((m,n),(m),(),[o](m))

and you call it with 3 pdls a(5,3,10,11), b(3,5,10,1,12), c(10) and an
output pdl d(3,11,5,10,12) (which can here not be automatically
created) as

func($a->thread(1,3),$b->thread(0,3),$c,$d->thread(0,1))

From the signature of func and the above call the pdls split into
the following groups of core, extra and thread dimensions (written in
the form pdl(core dims){thread dims}[extra dims]):

a(5,10){3,11}[] b(5){3,1}[10,12] c(){}[10] d(5){3,11}[10,12]

With this to help us along (it is in general helpful to write the
arguments down like this when you start playing with threading and
want to keep track of what is going on) we further deduce
that the number of explicit loop dimensions is 2 (by T1b from $a and $b)
with sizes (3,11) (by T2); 2 implicit loop dimensions (by T1a from $b
and $d) of size (10,12) (by T2) and the elements of are computed from
the input pdls in a way that can be expressed in pdl pseudo-code as

for (l=0;l<12;l++)
for (k=0;k<10;k++)
for (j=0;j<11;j++) effect of treating it as dummy dim (index j)
for (i=0;i<3;i++) |
d(i,j,:,k,l) = func(a(:,i,:,j),b(i,:,k,0,l),c(k))

Ugh, this example was really not easy in terms of bookkeeping. It
serves mostly as an example how to figure out what's going on when you
encounter a complicated looking expression. But now it is really time
to show that threading is useful by giving some more of our so called
"practical" examples.

[ The following examples will need some additional explanations in the
future. For the moment please try to live with the comments in the
code fragments. ]

# outer product by threaded multiplication
# stress that we need to do it with explicit call to my_biop1
# when using explicit threading
$res=zeroes(($a->dims)[0],($b->dims)[0]);
my_biop1($a->thread(0,-1),$b->thread(-1,0),$res->(0,1),"*");
# similar thing by implicit threading with auto-created pdl
$res = $a->dummy(1) * $b->dummy(0);

Example 3:

# different use of thread and unthread to shuffle a number of
# dimensions in one go without lots of calls to ->xchg and ->mv

# use thread/unthread to shuffle dimensions around
# just try it out and compare the child pdl with its parent
$trans = $a->thread(4,1,0,3,2)->unthread;

In this paragraph we are going to illustrate when explicit threading
is preferable over implicit threading and vice versa. But then again,
this is probably not the best way of putting the case since you already
know: the two flavours do mix. So, it's more about how to get the best
of both worlds and, anyway, in the best of Perl traditions: TIMTOWTDI !

[ Sorry, this still has to be filled in in a later release; either
refer to above examples or choose some new ones ]

Finally, this may be a good place to justify all the technical detail
we have been going on about for a couple of pages: why threading ?

Well, code that uses threading should be (considerably) faster than
code that uses explicit for-loops (or similar Perl constructs) to achieve
the same functionality. Especially on supercomputers (with vector
computing facilities/parallel processing) PDL threading will be
implemented in a way that takes advantage of the additional facilities
of these machines. Furthermore, it is a conceptually simply
construct (though technical details might get involved at times) and
can greatly reduce the syntactical complexity of PDL code (but keep
the admonition for documentation in mind). Once you
are comfortable with the threading way of thinking (and coding) it
shouldn't be too difficult to understand code that somebody else
has written than (provided he gave
you an idea what expected input dimensions are, etc.). As a general tip to
increase the performance of your code: if you have to introduce a loop
into your code try to reformulate the problem so that you can use
threading to perform the loop (as with anything there are exceptions
to this rule of thumb; but the authors of this document tend to
think that these are rare cases ;).

PDL:PP is part of the PDL distribution. It is used to generate
functions that are aware of indexing and threading rules from very
concise descriptions. It can be useful for you if you want to write
your own functions or if you want to interface functions from an
external library so that they support indexing and threading (and
maybe dataflow as well, see the PDL::Dataflow manpage). For further details
check the PDL::PP manpage.

A selection of signatures of PDL primitives to show how many
dimensions PP compiled functions gobble up (and therefore you can
figure out what will be threaded over). Most of those functions are
the basic ones defined in primitive.pd

Copyright (C) 1997 Christian Soeller (c.soeller@auckland.ac.nz) & Tuomas
J. Lukka (lukka@fas.harvard.edu) All rights reserved. Although destined for
release as a man page with the standard PDL distribution, it is not
public domain. Permission is granted to freely distribute verbatim
copies of this document provided that no modifications outside of
formatting be made, and that this notice remain intact. You are
permitted and encouraged to use its code and derivatives thereof in
your own source code for fun or for profit as you see fit.